The present invention relates to catalysts useful in the dehydrogenation of paraffins, and to methods for using the catalysts. The catalysts of the invention provide a combination of selectivity, thermal stability, and initial catalyst bed activity per unit volume that is highly advantageous. In one preferred embodiment, the invention relates to the dehydrogenation of substantially linear paraffins having between about 9 and 15 carbon atoms per molecule, and monoolefins derived from such paraffins find particular use in the production of biodegradable detergents. Through the use of the catalysts and methods of the invention, it is possible to obtain excellent reaction selectivity in a process that includes regeneration of the catalyst.
Many chemical processes that are practiced on a commercial scale involve the use of one or more catalysts for the production of intermediate or finished products. This is particularly the case in the petroleum-dependent arts. Because of the large volumes commonly processed, it is often possible for even incremental improvements in the performance of catalytic processes to provide commercially significant benefits. Examples of important catalytic hydrocarbon conversion processes include alkylation processes, hydrogenation processes, dehydrogenation processes, and isomerization processes.
Although catalysts by definition are not directly consumed by the chemical reactions that they promote, in the aforesaid and other processes catalysts are frequently rendered progressively less active during their use by one or more mechanisms known to those skilled in the art. In some cases, it is possible by taking certain steps such as coke removal, acid washing, or calcining to restore much of the lost activity so that the useful life of the catalyst is extended. Such steps are often referred to as xe2x80x9cregenerationxe2x80x9d of the catalyst. In general terms, it is highly desirable to employ catalysts that respond well to regeneration, in order to reduce the costs associated with catalyst replacement. However, in many processes catalyst regeneration is not a viable option. For example, a catalyst that might otherwise be regenerable by burning off accumulated coke might not have sufficiently high thermal stability to adequately withstand the high local temperatures that are generated under effective coke burning conditions.
The present invention is concerned with catalytic materials useful in the dehydrogenation of paraffins (saturated hydrocarbons). Dehydrogenation of paraffins is often carried out with the goal of introducing one or more olefinic linkages, either to produce an olefin product useful in and of itself, or to provide an effective xe2x80x9chandlexe2x80x9d on a molecule for subsequent reaction with some other species. The present invention is particularly concerned with the heterogeneously catalyzed dehydrogenation of detergent range paraffins (paraffins with carbon numbers in the 9-15 range) to obtain products that contain a single unsaturated linkage per molecule (monoolefins). The resulting monoolefins (detergent range monoolefins) are useful for reaction with a second organic species that includes an aromatic nucleus to produce alkylbenzenes. Such alkylbenzenes having substantially linear alkyl substituents attached to benzene rings are useful for conversion to alkylbenzene sulfonates that are employed in detergent formulations in both the industrial and consumer products markets. Alkylbenzenes derived primarily from linear paraffins are particularly advantageous in the production of detergents, since their sulfonates possess a very high degree of biodegradability. The term xe2x80x9csubstantially linearxe2x80x9d as used herein means that the type and degree of branching present in the paraffin that is to be dehydrogenated to obtain olefins for subsequent use in alkylbenzene sulfonate production are limited to those which provide an alkylbenzene sulfonate with a degree of biodegradability that is acceptable according to current standards promulgated by industry and regulatory agencies. Alkylbenzenes containing a single alkyl substituent attached to a benzene ring (monoalkylbenzenes) are advantageous, as is known in the art, since they tend to provide favorable detergent performance characteristics. Alkylbenzene mixtures consisting primarily of monoalkylbenzenes with linear alkyl substituents are also recognized as advantageous, and they are the types most widely used by the detergent industry. Such mixtures are commonly referred to as xe2x80x9clinear alkylbenzenexe2x80x9d or xe2x80x9cLABxe2x80x9d by those skilled in the art.
Production of monoolefins in a dehydrogenation process typically involves the contacting of saturated hydrocarbons with a suitable catalyst under reaction conditions adjusted to favor monoolefin formation. However, the production of monoolefin is inevitably accompanied by some formation of undesirable by-products such as diolefins, aromatics, and cracking products. The amount of diolefin formed depends mainly upon the paraffin structure and the conversion level, and relatively little control of diolefin formation is possible by means of the other reaction conditions. The formation of cracking products can be minimized by using a nonacidic catalyst and by avoiding extremely high temperatures. Aromatics formation is significantly influenced by both the selectivity of the catalyst and the reaction conditions employed. It is well known in the art that great economic advantages can be realized by using a highly selective dehydrogenation catalyst that minimizes the formation of aromatics at a given level of paraffin conversion. Specific advantages associated with lower aromatics formation include lower paraffin consumption, lower consumption of monoolefin by side reactions with aromatics during alkylbenzene production, higher recycle paraffin purity, and less extensive catalyst inhibition and fouling.
Many catalysts useful for the dehydrogenation of paraffins to olefins are known in the art. Typically, known catalyst materials comprise one or more active metals or metal oxides in a finely divided form, deposited upon the surface of particles of a relatively inert carrier substance such as a silica or an alumina. Alternative means known in the art by which the primary catalytic component(s) or precursors thereof may be rendered into the required finely divided state upon the surface of a suitably pretreated support include such methods as precipitation, adsorption from an aqueous solution, and ion exchange techniques that make use of Zeolite(copyright) (molecular sieve) carrier materials. Typically, following the deposition of one or more species onto a selected support to provide a raw catalyst, the raw catalyst material is subjected to some sort of heat treatment at an elevated temperature for a suitable time, often in the presence of a controlled atmosphere, which may be inert, oxidizing, or reducing. The prior art is replete with examples of aluminas and silicas of various particle sizes, crystalline phases, pore structures, etc., combined with a very broad variety of other components deposited upon their surfaces. In many cases, the deposited components comprise at least one primary catalytic component and at least one additional component such as an activator, attenuator, or modifier.
In general terms, the performance of a catalyst is largely determined by three critical properties that are readily observable and known to those skilled in the art of catalysis. These properties are 1) selectivity, 2) activity, and 3) thermal stability.
In the case of paraffin dehydrogenation to produce monoolefin, the selectivity of a catalyst is a measure of its ability under appropriate reaction conditions to maximize the fraction of the total converted paraffin that is converted to monoolefin. Since higher formation of each unwanted by-product necessarily results in lower formation of monoolefin at a given paraffin conversion, selectivity is improved if by-product formation is reduced at a given paraffin conversion. Thus, comparisons of catalyst selectivity can be made in terms of the amounts of by-products formed at equal paraffin conversion in runs that use different catalysts but are essentially equivalent in terms of the other reaction conditions. If different low acidity catalysts are being compared, the most important difference will typically be between the amounts of aromatics formed at a given paraffin conversion. In comparing selectivities within a series of alternative catalysts, it is particularly convenient to express the selectivities in comparison to a single standard catalyst. Thus, for each alternative catalyst that exhibits a selectivity improvement, the size of the improvement can be expressed as the percentage by which the alternative catalyst reduces the formation of aromatics at a given paraffin conversion under standard reaction conditions in comparison to the standard catalyst.
In the case of paraffin dehydrogenation, the activity of a catalyst is a measure of its ability to promote paraffin conversion. In a continuous process under any particular reaction conditions, higher catalyst activity results in higher conversion over a given amount of catalyst. For practical purposes, the most important measure of catalytic activity is the volumetric activity, meaning the activity per unit volume of catalyst bed. Under given continuous reaction conditions, a catalyst with higher volumetric activity is able to provide higher paraffin conversion over a catalyst bed of a given volume. Alternatively, it is able to reduce the catalyst bed volume (reactor size) required to produce a given paraffin conversion. Factors which significantly affect the volumetric activity of a catalyst include the surface area, the bulk density, the types and weight percentages of the included active metals, the distributions of the active metals within the support pellets, and the degree of diffusion resistance associated with the pore structure. Since dehydrogenation catalysts lose activity during normal use, comparisons of the activity of different catalyst types must be made at comparable degrees of catalyst deactivation. This can be done by comparing paraffin conversion ranges for runs of equal length that begin with fresh catalyst and employ standard reaction conditions.
A dehydrogenation catalyst must have a high degree of thermal stability in order to hold up adequately under the elevated temperatures encountered during its normal use. High thermal stability is particularly important if the catalyst will be regenerated by burning off accumulated coke, a procedure that tends to produce unusually high local temperatures. A deficiency of thermal stability results in an excessive loss of activity during exposure of the catalyst to high temperatures. One process that contributes to activity loss during thermal exposure involves the agglomeration (coalescence) of particles of the active component(s). Another process involved is degradation of the support structure in such a way that some catalytic particles become entrapped in inaccessible locations within surrounding layers of support material. In either case, the amount of catalytic surface available to the reaction is reduced. The thermal stability of a particular catalyst can be determined by comparing the activities of representative samples from the same lot that have and have not been exposed to a suitable high temperature aging treatment.
It is known that in the catalytic dehydrogenation of detergent range paraffins the percentage conversion to monoolefins in a single pass through the reactor is subject to an equilibrium constraint. While the limiting conversion can vary considerably under various reaction conditions, the actual percentage of monoolefin in the products is typically not greater than about twenty weight percent. It is also well known that the formation of monoolefin in such processes is accompanied by the formation of various less desirable by-products including diolefins, aromatics, and hydrocarbons with carbon numbers below the detergent range which are formed by cracking reactions. As used herein, the term xe2x80x9cconversionxe2x80x9d means the weight percentage of the detergent range paraffin in the feed that is converted in a single pass to species other than paraffins within the same carbon number range. In cases in which the feed contains species other than detergent range paraffins, these components of the feed are ignored in the calculation of conversion and selectivity. In general, higher conversion and higher selectivity are advantageous, but an increase in conversion tends to lower selectivity.
A well known problem encountered in the production of detergent range olefins by catalytic dehydrogenation of paraffins is the loss of catalyst activity during paraffin processing. The catalyst can lose activity as a result of strong catalyst poisons such as sulfur compounds in the feed, and such activity loss is generally controlled by controlling feed purity. However, even when the feed contains extremely low levels of such poisons, the catalyst tends to deactivate at a significant-rate due to the formation of coke on the catalytic surfaces. The rate of coke formation can vary widely depending upon the combination of reaction conditions selected. In general, a lower rate of coke formation is advantageous since this reduces various costs associated with catalyst regeneration or replacement and facilitates the maintenance of both conversion and other reaction conditions within optimum ranges for extended periods of operation.
One method used in prior art paraffin dehydrogenation processes to reduce catalyst deactivation is to mix varying amounts of hydrogen with the vaporized paraffin feed prior to its introduction into the catalytic reaction zone. It is taught in U.S. Pat. No. 4,343,724 for example that such hydrogen serves a xe2x80x9cdual-functionxe2x80x9d in both diluting the paraffin and xe2x80x9csuppressing the formation of hydrogen-deficient, carbonaceous depositsxe2x80x9d upon the catalyst. In many cases, the amount of added hydrogen used in patent examples has been extremely large, for example 4-8 moles of hydrogen per mole of hydrocarbon. Severe disadvantages accompany such large additions of hydrogen, including an adverse effect upon the equilibrium for monoolefin formation, increased size of most portions of the processing equipment for a given production rate, and increased energy and maintenance costs associated with the recovery, compression, and recycle of hydrogen. Thus, it is greatly advantageous to reduce the hydrogen to hydrocarbon mole ratio (H2:HC ratio) used in the process. U.S. Pat. No. 5,324,880 teaches the use of H2:HC ratios within the range 0.5-1.9, and even lower ratios such as those within the range 0.3-0.5 are useful under some circumstances. However, it appears that some added hydrogen is always necessary in order to maintain the catalyst in an active state.
Typically, the activity of a paraffin dehydrogenation catalyst declines during use until the remaining activity is insufficient to support further economical operation without prior replacement or regeneration of the catalyst. Since the cost of a fresh catalyst charge for an industrially sized reactor system can easily reach into the hundreds of thousands of dollars, it is most desirable to extend the useful life of a given bed of catalyst by regenerating it one or more times prior to its eventual replacement. Costs reduced by catalyst regeneration include those for acquisition of new catalyst, down time and labor associated with catalyst replacement, processing of spent catalyst for precious metal recovery, and replacement of precious metals lost during spent catalyst processing. Furthermore, the necessity of the use of costly equipment for catalyst addition without shutdown of a reactor can be avoided, and occasional episodes of catalyst poisoning are less costly since regeneration of the catalyst is often sufficient to restore normal operation.
The ability of a catalyst to be effectively regenerated is commonly referred to as the regenerability of the catalyst. In order to be regenerable, a catalyst must have a high degree of thermal stability so that activity losses by thermal degradation are minimized during high temperature regeneration procedures such as the burning of accumulated coke. Since some loss of activity during each regeneration is inevitable, another requirement for regenerability is a sufficient amount of activity in the fresh catalyst to compensate for the activity losses incurred during regenerations. A catalyst that is highly regenerable is able to retain an adequate level of activity throughout a series of many operating cycles and intervening regenerations. No laboratory test can completely quantify the degree of regenerability of a catalyst. However, a useful indication of regenerability can be obtained in the laboratory by measuring the initial activity and the thermal stability at a temperature representative of the intended regeneration procedure.
In a process for the production of monoolefins by dehydrogenation of detergent range paraffins, great economic advantages can be realized through the use of a catalyst that has favorable characteristics with regard to selectivity, volumetric activity, thermal stability, and regenerability. In practice, however; previously known catalysts have been deficient in at least one of these properties. Catalysts that have been regenerable have been deficient in selectivity, and catalysts with relatively high selectivity have been deficient in volumetric activity, thermal stability, or regenerability. Thus, the discovery of a catalyst with favorable characteristics with regard to all four of these properties as described herein represents a major advance in the dehydrogenation of paraffins and in the manufacture of alkylbenzene for use in the detergent industry.
The prior art associated with catalytic reactions involving hydrocarbons includes U.S. Pat. Nos. 3,484,498; 3,494,971; 3,696,160; 3,655,621; 3,234,298; 3,472,763; 3,662,015; 4,409,401; 4,409,410; 4,523,048; 3,201,487; 4,358,628; 4,489,213; 3,751,506; 4,387,259; and 4,409,412, the entire contents of which are herein incorporated by reference thereto. Prior art patents directed at catalysts useful for dehydrogenation of hydrocarbons include U.S. Pat. Nos. 3,274,287; 3,315,007; 3,315,008; 3,745,112; and 4,430,517, the entire contents of which are herein incorporated by reference thereto.
The prior art associated with the dehydrogenation of detergent range paraffins to form monoolefins includes U.S. Pat. No. 3,761,531, the entire contents of which are incorporated herein by reference. In this patent is described a dehydrogenation method comprising contacting a hydrocarbon at dehydrogenation conditions with a catalytic composite comprising a combination of catalytically effective amounts of a platinum group component, a Group IV-A metallic component, a Group V-A metallic component, and an alkali or alkaline earth metallic component with an alumina carrier material. It is taught therein that the preferred alumina carrier material has a relatively low apparent bulk density, with a bulk density in the range of about 0.3 to about 0.4 g/cm3 being especially preferred, and a bulk density of about 0.33 g/cm3 to be used for best results. While the catalysts described in U.S. Pat. No. 3,761,531 exhibit an acceptable level of selectivity towards the desired reaction, they generally display relatively poor volumetric activity and/or thermal stability. Consequently, such catalysts are not considered to be regenerable. In typical practice, after a single reaction cycle, a bed of such catalyst is replaced with fresh catalyst. Such recharging is expensive both from the perspective of the cost of the catalyst and in terms of the reactor down time experienced.
Other patents which relate to catalysts and processes useful in the dehydrogenation of detergent range paraffins to form monoolefins are U.S. Pat. Nos. 3,585,253; 3,632,662; 3,920,615; and 5,324,880, the entire contents of which are herein incorporated by reference thereto. The catalysts described in U.S. Pat. No. 3,920,615 have acceptable selectivity, but they were found to be deficient in volumetric activity and/or regenerability. The catalysts described in the other three patents mentioned above rank highly in terms of volumetric activity and regenerability, but they are lacking in that their selectivity is undesirably low.
The following US Patents, the entire contents of each of which are herein incorporated by reference, are useful in illustrating the differences between the prior art and the instant invention: 3,293,319; 3,448,165 (especially col. 5, lines 26-33); 3,576,766 (esp. col. 5, lines 31-60); 3,647,719 (esp. col. 4, line 68-col. 5, line 4); 3,649,566 (esp. col. 5, lines 13-24); 3,761,531 (esp. col. 4, line 68-col. 5, line 17); 3,767,594 (esp. col. 2, lines 46-60 and Example I); 3,825,612 (esp. col. 5, lines 26-38); 3,998,900 (esp. col. 5, line 60-col. 6, lines 3); 4,048,245 (esp. col. 6, lines 39-51); 4,070,413 (esp. Example I); 4,125,565 (esp. col. 6, lines 38-51); 4,136,127 (esp. col. 6, lines 41-54); 4,172,853 (esp. col. 6, line 61-col. 7, lines 6); 4,177,218 (esp. Example I and col. 3, line 56-col. 4, line 14); 4,207,425 (esp. col. 6, lines 33-54); 4,216,346 (esp. col. 6, lines 40-54); 4,227,026 (esp. col. 6, lines 36-50); 4,268,706 (esp. col. 6, lines 38-52; col. 7, line 27-col. 8 line 59; and col. 19, lines 3-10); 4,312,792 (esp. col. 6, line 63-col. 7, line 9; col. 7, line 54-col. 9, line 19; and col. 19, lines 22-28); 4,341,664 (esp. col. 6, line 62-col. 7, line 8; col. 7, line 53-col. 9, line 18; and col. 19, lines 1-8); 4,343,724 (esp. col. 6, line 61-col. 7, line 7; col. 7, line 52-col. 9, line 17; and col. 19, lines 14-21); 4,396,540 (esp. col. 6, line 61-col. 7, line 7; col. 7, line 52-col. 9, line 17; and col. 19, lines 5-11); 4,486,547 (esp. col. 6, line 56-col. 7, line 23); 4,551,574 (esp. col. 6, line 60-col. 7, line 25); 4,595,673 (esp. col. 6, lines 15-43); 4,608,360; 4,677,237 (esp. col. 6, lines 25-33); and 4,827,072 (esp. col. 10, line 31-col. 11, line 11). These patents are believed to be assigned to UOP, LLC. With the exception of U.S. Pat. No. 4,070,413, these prior art patents have claims limited to the inclusion of one or more elements other than platinum group metals, Group I-B metals, and alkali metals. The remaining patent U.S. Pat. No. 4,070,413 has claims limited to the use of a particular steam-treated alumina support. In each of these patents, the teaching concerning the shape and size of the catalyst particles is that {fraction (1/16)} inch spheres are preferred. A few of these patents (U.S. Pat. Nos. 4,268,706; 4,312,792; 4,341,664; 4,343,724; and 4,396,540) teach that {fraction (1/16)} inch extrudates are also preferred. All examples used {fraction (1/16)} inch spheres, and there is no indication in these patents that extrudates are ever preferred over spheres. Only one of the patents, U.S. Pat. No. 4,608,360, discusses pore size distribution, and it teaches that more than 55% of the total pore volume should be contained in pores with diameters of 600 Angstroms or larger. Higher selectivity was attributed to such a pore structure in Example III of that patent. The teaching concerning average pore size is inconsistent and not very specific. The ranges mentioned for average pore diameter include 20-30, 20-300, and 20-3000 Angstroms. The most preferred bulk densities for spheres were below 0.5 g/cm3 in some of the earlier-issued of these patents, and near 0.3 g/cm3 in all of the remaining patents. Bulk density ranges indicated for extrudates were 0.4-0.85 or 0.5-0.85 g/cm3.
The following US Patents, the entire contents of each of which are herein incorporated by reference, are also useful in illustrating the differences between the prior art and the instant invention: 5,677,260 (esp. col. 4, lines 50-59); 3,458,592; 3,662,018; 3,527,836; 3,274,287 (esp. col. 3, line 66-col. 4, line 20 and Example IV); 3,315,007 (esp. col. 3, lines 25-56 and Example I); 3,315,008 (esp. col. 3, lines 12-44); 3,585,253; 3,632,662 (esp. col. 2, lines 50-61 and col. 3, lines 26-31); 3,920,615; and 5,324,880. The catalysts disclosed in U.S. Pat. No. 5,677,260 believed to be assigned to Indian Petrochemicals include an unusually large number of added elements, and they closely resemble various catalysts disclosed in patents believed to be assigned to UOP, LLC. A preference for {fraction (1/16)} inch spheres with bulk density near 0.3 g/cm3 is indicated therein. The preferred pore distribution is said to be xe2x80x9cmesoporousxe2x80x9d; however, no further definition is included. U.S. Pat. Nos. 3,458,592; 3,662,018; and 3,527,836 believed to be assigned to Texaco and British Petroleum claim catalysts with molecular sieve supports.
Among the listed patents originally assigned to Monsanto Company, the earliest ones: 3,274,287; 3,315,007; and 3,315,008 do not mention the use of copper in combination with platinum and a support, while the later ones: 3,585,253; 3,632,662; 3,920,615; and 5,324,880 disclose such. These patents teach that the macropore volume (the volume contained in pores with average diameters above 700 Angstroms) should be at least 0.05 cm3/g and that higher macropore volumes are preferred. They say nothing about bulk density. In U.S. Pat. No. 3,920,615, it is taught that selectivity is improved by calcining to a surface area of less than 150 m2/g. Although such calcination affects the pore structure, no particular final pore structure is defined simply by specifying the surface area. The allowed variations in starting materials and order of operations significantly affect the relationship between surface area and pore structure.
Although the prior art patents set forth and described above in the Background Information section herein contain a wealth of information concerning the composition and use of various catalysts useful in the dehydrogenation of paraffins, there is nothing in the prior art that points to the conclusion or even suggests that a high degree of volumetric activity, thermal stability, and selectivity heretofore unseen would be provided by a catalyst according to this invention which comprises one or more of the elements: platinum, rhodium, iridium, palladium, ruthenium, and osmium (the xe2x80x9cplatinum group elementsxe2x80x9d) deposited upon a porous alumina support selected to provide in the finished catalyst a surface area greater than 100 m2/g, a volume of pores with diameters below 60 Angstrom units that is less than 0.05 cm3/g, a volume of pores with diameters in the range of 60-350 Angstrom units that is greater than 0.50 cm3/g, and a volume of pores with diameters in the range of 60-350 Angstrom units that is greater than 70% of the total contained pore volume. In a preferred form of the invention, the volume of pores with diameters in the range of 60-350 Angstrom units is greater than 75% of the total contained pore volume. In a preferred form of the invention, the packed bulk density of the catalyst is greater than 0.50 g/cm3. In fact, the prior art points to just the opposite conclusion; that such a catalyst would possess relatively low activity and selectivity, due to a deficiency of pores with diameters greater than 600 or 700 Angstroms. Therefore, the beneficial results obtained through use of the catalysts as described further below according to the instant invention were wholly unexpected.
The present invention concerns a catalyst useful in the dehydrogenation of paraffinic hydrocarbons which in one form comprises a porous aluminum oxide support and a primary catalytic component comprising one or more elements selected from the group consisting of: platinum, palladium, osmium, ruthenium, iridium, and rhodium disposed upon the support, said catalyst having a surface area greater than 100 m 2/g, a packed bulk density greater than 0.50 g/cm3, a volume of pores with diameters below 60 Angstrom units that is less than 0.05 cm3/g, a volume of pores with diameters in the range of 60-350 Angstrom units that is greater than 0.50 cm3/g, and a volume of pores with diameters in the range of 60-350 Angstrom units that is greater than about 70% of the total contained pore volume.
The catalysts taught herein possess a combination of thermal stability and volumetric activity that is essentially equal to that provided by the most stable and active prior art catalysts useful in producing detergent range monoolefins from detergent range paraffins. Thus, one of the advantages of catalysts prepared according to the teachings herein is that they are especially well suited for use in a process that includes catalyst regeneration. A further advantage is that they provide better selectivity than prior art catalysts that have had comparable thermal stability and volumetric activity. Thus, the advantages of catalyst regeneration recited above can be realized in combination with excellent catalyst selectivity for the first time ever in the production of detergent range monoolefins from paraffinic starting materials. Further, in any dehydrogenation process in which the catalysts according to this invention are utilized, regardless of whether the process includes catalyst regeneration, the high volumetric activity of the catalysts prepared according to the invention can be exploited to obtain longer reaction cycles or higher average conversion in a reactor of a given size. Alternatively, reactor size can be reduced without any sacrifice of average production rate. Further, any necessary fine-tuning of the volumetric activity can easily be accomplished by varying the Pt loading. Such adjustments are well within the level of skill of one of ordinary skill in the art. The high catalyst selectivity provided by the catalysts of the invention is clearly advantageous since it can be exploited to obtain either more production at a given raw material cost per pound of product or lower raw material consumption at a given production rate. In a preferred embodiment, the catalysts of the invention provide both high catalyst selectivity and a relatively low pressure drop. A low pressure drop tends to enhance reaction selectivity by providing a lower average reaction pressure, and it may also allow the total energy consumption associated with a process to be reduced under some circumstances.
This invention is concerned with the formation of monoolefins by catalytic dehydrogenation of paraffins having between 9 and 15 carbon atoms per molecule. Monoolefins so formed may subsequently be employed in the manufacture of alkylbenzene-based detergent compositions. The catalysts according to the present invention utilize known metals or combinations thereof as primary catalytic components for the dehydrogenation of paraffins, in combination with a porous alumina support selected to provide in the finished catalyst a unique specified combination of structural characteristics. In the preferred embodiment, the primary catalytic component is platinum.
The catalysts according to the invention are supported catalysts, i.e., they comprise at least one active catalytic material that is supported on an inert carrier (the xe2x80x9ccatalyst supportxe2x80x9d). In accordance with this invention, the catalyst support is a porous alumina selected to provide in the finished catalyst specified physical properties that have been found to provide advantageous catalyst performance characteristics relating to selectivity, activity, and regenerability. The specified physical properties in a finished catalyst according to the invention include limitations upon the microstructure of the finished catalyst, including its surface area and pore structure.
It is well known that the microstructure of a finished catalyst is dependent upon the initial properties of the support material in its raw state prior to its exposure to reagents, conditions, and operations employed throughout the catalyst preparation process, some of which are well known to those in the catalyst preparation arts including impregnation with catalytically active metal(s), calcination steps, hydrothermal treatments, etc. The microstructure of the starting alumina may be altered to a considerable extent during catalyst preparation. Thus, the selection of a starting alumina for the preparation of a catalyst in accordance with the invention must properly be based upon a specification that characterizes the microstructure of the resulting finished catalyst. A starting alumina for use in catalysts prepared in accordance with the teachings of the invention can be from any source and can be made by any method, provided that the resulting finished catalyst has the unique set of physical properties specified herein. It is most preferable to use a starting alumina with a relatively narrow pore size distribution. Various methods for producing alumina with a controlled and narrow pore size distribution are known. Some of these methods have been described by D. L. Trimm and A. Stanislaus in Applied Catalysis 21, 215-238 (1986), the entire contents of which are incorporated herein by reference thereto. An especially preferred starting alumina for preparing catalysts according to this invention and obtaining the required combination of physical properties set forth herein is the type produced by Engelhard Corporation of Iselin, N.J. having the grade designation xe2x80x9cAE-30xe2x80x9d.
Catalysts prepared in accordance with the invention have exhibited excellent selectivity in combination with a very low content of pores with diameters greater than 600 Angstrom units. For example, in catalysts B, C, E, and F described in Table I below, such pores contribute less than 10.5% of the total pore volume. According to the invention, the starting support is selected to provide a finished catalyst structure that preferably has a very low volumetric content (less than 20.00% of the total contained pore volume, including every hundredth percentage between 20.00% and 0.00%) of pores with diameters greater than 600 Angstrom units. Based upon the teachings of the prior art of this field, such a pore structure would be expected to result in undesirably low catalyst selectivity. Thus, the high selectivity obtained in accordance with the invention in the absence of a substantial amount of such large pores was wholly unexpected.
The relatively high packed bulk density (greater than 0.50 g/cm3, and including every hundredth g/cm3 between 0.50 and 0.80) preferred in catalysts according to one form of the invention is highly advantageous because of its favorable effect upon volumetric activity. While some prior art catalysts have had packed bulk densities comparable to those of the present invention, such prior art catalysts have been deficient in activity per unit weight, selectivity, or thermal stability. Thus, the fact that the preferred catalysts prepared in accordance with the invention have relatively high packed bulk density while maintaining excellent activity per unit weight, selectivity, and thermal stability represents a significant improvement in the art.
Catalysts prepared according to the invention contain a primary catalytic component comprising one-or more elements selected from the group consisting of: ruthenium (Ru), rhenium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), (xe2x80x9cplatinum group metalsxe2x80x9d). While the prior art related to dehydrogenation is fraught with patents that include claims to the use of one or more of the platinum metals in one form or another disposed upon various supports, Pt is the only metal that has been used commercially to any appreciable extent as the primary catalytic component for the dehydrogenation of detergent range paraffins. According to the present invention, it is preferred to include platinum, and it is most preferred to employ platinum and platinum alone as the primary catalytic component. However, according to the present invention, it is also possible to use other platinum group metals either alone or in various combinations as the primary catalytic component of a catalyst according to the invention. Although preferred amounts of primary catalytic component expressed as a weight percentage based on the total weight of the finished catalyst are herein specified for preferred embodiments of this invention, the use of any amount of primary catalytic component between 0.01% and 3.00% by weight based upon the total weight of the finished catalyst, including every hundredth percentage therebetween, is embraced by the scope of this invention. In any case, the primary catalytic component is disposed upon the support in such a distribution to provide catalytic surfaces that are readily accessible to the reaction mixture.
When the primary catalytic component is platinum, the platinum content of the finished catalyst expressed in units of weight percent based upon the total weight of the finished catalyst is variable and is preferably in the range of about 0.02% to 2.00%, including every hundredth percentage therebetween, more preferably between about 0.20% and 1.00%, including every hundredth percentage therebetween, more preferably still between about 0.40% and 0.70%, including every hundredth percentage therebetween, with 0.55% being most preferred. Such levels can be obtained by one of ordinary skill in the art without resorting to undue experimentation, as methods for alteration of this art-recognized variable are well known.
While catalysts prepared in accordance with the present invention may include only a support and a primary catalytic component, inclusion of an activator component which functions to enhance catalyst performance characteristics is preferable. A suitable activator component may be selected from one or more of the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, silver, lanthanum, hafnium, tantalum, tungsten, rhenium, and gold. However, it is especially preferable to include an activator component that comprises one or more metals selected from the elements copper, silver, and gold (Group I-B). The primary function of an activator component as employed herein is to enhance the activity and/or selectivity of the catalyst by means of various effects upon the primary catalytic component. For example, an activator component can be used to improve the dispersion of the primary catalytic component and/or to improve the distribution of the primary catalytic component within the catalyst support pellets. To achieve the desired effect of improving catalyst activity and selectivity, the activator component must be deposited onto the catalyst support. Regardless of the activator component selected, a catalyst according to the invention may comprise any amount of activator component between 0.10% and 5.00% based upon the total weight of the finished catalyst, including every hundredth percentage therebetween. However, it is most preferred to use copper and copper alone as the activator component since copper is both highly effective as an activator and relatively low in cost. When copper is, used, the concentration of copper in the finished catalyst material, expressed as a weight percentage of the total catalyst, is preferably in the range of about 0.10%-5.00%, including every hundredth percentage therebetween, more preferably between about 0.50% and 4.00%, including every hundredth percentage therebetween, more preferably still between about 1.00% and 3.00%, including every hundredth percentage therebetween, with about 2.00% being most preferable. Such levels in the finished catalyst are easily achieved by those of ordinary skill in the art without resorting to undue experimentation, through use of conventional techniques used to deposit metals onto catalyst supports as more fully described in the Examples.
In order to provide a catalyst according to the invention which has the most favorable combination of selectivity, activity, and regenerability, it is necessary to control the acidity level of the final catalyst. If the acidity of a dehydrogenation catalyst according to this invention is too high, acid-catalyzed side reactions such as cracking and isomerization will be promoted in the dehydrogenation process to an extent that detracts from processing economics and operating efficiency. According to one form of the invention, alkali metals or mixtures thereof are preferred as acidity control agents since the oxides of these elements are basic in nature and highly effective for neutralizing the effects of various acidic species that are commonly encountered in or on the catalyst. Other metals whose oxides are known to possess an alkaline character such as the alkaline earth metals can also be used as acidity control agents, but they are less preferred than the alkalis, since they are generally less effective. Regardless of the acidity control agent(s) selected, the acidity control agent(s) are preferably added to the catalyst during its preparation using means known to those skilled in the art and as mentioned herein, and the amount of acidity control agent present on the finished catalyst may be any amount between 0.001% and 1.000% by weight based upon the total weight of the finished catalyst, including every thousandth percentage therebetween.
Some available starting aluminas already contain enough alkali, such as sodium for instance, as an impurity to provide an effective level of acidity control. Thus, the addition of an acidity control agent to the starting alumina during catalyst preparation may not be necessary in all cases. However, it is normally preferred to include an added acidity control component comprising one or more of the alkali metals in order to provide a greater degree of protection against potential catalyst acidification. If an added acidity control component is used, it is preferably deposited onto the surface of the support during catalyst preparation, using similar or the same techniques as are known in the art to be useful for depositing metals onto catalyst supports.
According to a preferred form of the invention, it is especially desirable to use added potassium as an acidity control component. When added potassium is used, the amount is preferably in the range of about 0.01% -2.00%, including every hundredth percentage therebetween, more preferably in the range of about 0.05%-1.00%, including every hundredth percentage therebetween, and more preferably still in the range of about 0.10%-0.60%, including every hundredth percentage therebetween, with about 0.20% being most preferable. In this specification and the appended claims, the amount of added potassium is expressed for convenience as a percentage by weight of metal based upon the total final catalyst weight, even though the potassium is expected to be present in the catalyst in the form of an oxide or salt. If other alkali metals or mixtures of alkali metals are added in place of potassium, appropriate amounts thereof can be readily determined using equivalent weights of the element(s) to be used without resorting to undue experimentation.
It is known to those of ordinary skill in the art of catalysis that the size and shape of catalyst pellets can be varied. Two of the most common forms in widespread use are spheres and extrudates. Catalysts previously used commercially for the dehydrogenation of detergent range paraffins have used essentially spherical porous alumina supports with average pellet diameters ranging from about {fraction (1/16)} inch to about xe2x85x9 inch. Known methods for the production of spherical alumina pellets include such methods as the agglomeration of wetted alumina powder and the oil drop method (formation of hydrogel spheres from a fluid precursor dropped into a heated oil bath). Extrudates are prepared by converting powdered alumina into an extrudable dough by various known methods and then extruding the dough through a die under suitable conditions. As the extrudate emerges from the die, it can be cut to any desired length using, for example, a rotating or reciprocating knife means at the exit point of the extruder. Extrudates having a wide variety of cross sections can be obtained by varying the shape of the die. For example, circular or trilobed cross sections are but two profiles often produced.
It is well known that pellet size and pellet shape have important effects upon the performance of a catalyst. Shorter diffusion pathways within the pellets are typically associated with higher catalyst selectivity, and the diffusion pathways can be shortened by using smaller pellets and/or by using pellet shapes that have larger surface to volume ratios. However, the effects of pellet size and shape upon the mechanical strength of the pellets and the pressure drop across a catalyst bed must also be taken into consideration. For example, smaller catalyst pellets result in a higher pressure drop, and pellet shapes other than spherical tend to result in lower mechanical strength. Thus, the optimization of pellet size and shape for a given process often requires a balancing of effects upon selectivity, pressure drop, and catalyst durability. The most favorable balance can vary significantly as the processing conditions are varied. However, for any particular case, the best balance is readily determinable by those skilled in the art through routine experimentation.
Catalyst pellets of any size and shape useful for the dehydrogenation of detergent range paraffins can be employed according to the teachings herein in the practice of the invention. If spheres are used, the preferred diameter range of the spheres is between 1.0 and 4.0 millimeters, including every tenth millimeter therebetween, with 2.5 mm being most preferable. However, for purposes of the invention, it is more preferred to use extrudates. It is preferred to use an extrudate with its longest length dimension in the range of between 1.0 and 10.0 millimeters, including every tenth millimeter therebetween. It is more preferred to use such an extrudate having an approximately circular cross section, a diameter of between about 1.0 and 4.0 millimeters, including every tenth millimeter therebetween, and a length sufficient to provide an average length to diameter ratio in the range of about 1-4. Such extrudates having an average diameter near 1.60 millimeters and an average length to diameter ratio in the range of about 2.00-4.00 (including every hundredth therebetween) are especially preferred, with an average length to diameter ratio of 3.00 being most preferred.
A finished catalyst provided in accordance with this invention has a surface area that is greater than 100 m2/g and is preferably within the range 120-200 m2/ g, including every integral m2/g therebetween. The range 135-150 m2/g, including every integral m2/g therebetween, is especially preferred.
Catalysts according to the invention may be conveniently characterized as possessing a specified volume of pores whose average diameters fall within a first range of diameters, and another specified volume of pores whose average diameters fall within a second range of diameters. The volume of pores within the second range of diameters may be further characterized in terms of the percentage of the total pore volume falling within said second range of diameters. In a catalyst according to the invention, the volume of pores having diameters below 60 Angstroms is less than 0.05 cm3/g, with a volume of pores having diameters below 60 angstroms of less than 0.02 cm3/g being more preferable, and with a volume of pores having diameters below 60 angstroms of less than 0.01 cm3/g being most preferred. The volume of pores having diameters in the range 60-350 Angstroms is greater than 0.50 cm3/g, and is more preferably in the range of 0.60-0.80 cm3/g, including every hundredth cm3/g therebetween, with about 0.69 cm3/g being most preferred.
When expressed as a percentage of the total pore volume present in a catalyst according to one embodiment of the invention, the volume of pores with diameters in the range 60-350 Angstroms is greater than 75.00%. More preferably, the volume of pores with diameters in the range 60-350 Angstroms is greater than 80.00%. More preferably still, the volume of pores with diameters in the range 60-350 Angstroms is greater than 84.00%, with the range 86.00-89.00%, including every hundredth percentage therebetween, being the most preferable. Pore volume percentages herein are expressed as a percentage of the total pore volume of the catalyst. Measurements of pore volumes are determined by the mercury intrusion method, such method being known to those of ordinary skill in the catalyst art.
According to the conventional wisdom of the prior art, catalysts having the combination of properties (including pore-distributions) possessed by the catalysts of this invention would have been; expected to exhibit undesirably low catalyst selectivity. Moreover, they would have been expected to have unacceptably high diffusion resistance due to a failure to provide an adequate content of pores with diameters above about 600 or 700 Angstroms. Thus, it was surprising that catalysts according to the invention were found to have excellent selectivity with an attendant low content of large pores.
Other physical properties are useful in further characterization of the catalysts of this invention, including the packed bulk density. In a preferred embodiment, a catalyst in accordance with the invention has a packed bulk density greater than 0.50 g/cm3. A packed bulk density in the range 0.50-0.65 g/cm3, including every hundredth g/cm3 therebetween, is especially preferred, with a packed bulk density of 0.57 g/cm3 being most preferred. Packed bulk densities above 0.50 g/cm3 tend to place limits upon both pellet density and catalyst bed void volume that tend to have favorable effects upon the volumetric activity of the catalyst.
Packed bulk densities are stated herein on the basis of measurements made by the following method. Catalyst in an amount in the range of 900-1000 ml is poured into a tared, vibrated, 1000 ml graduated cylinder. Vibration of the cylinder is continued until the volume becomes constant. The final volume of the catalyst sample is read, and the weight of the catalyst sample is determined. Packed bulk density is calculated by dividing the sample weight by the final sample volume. The measurements are made on samples that have been protected from atmospheric moisture following their final calcination during catalyst preparation.
Another characteristic of catalysts according to the invention. is that they are regenerable. For purposes of this specification and the appended claims, a catalyst is xe2x80x9cregenerablexe2x80x9d if its response to a practical regeneration procedure is sufficiently favorable to make it economically advantageous to regenerate the catalyst at least once during its useful life. The degree of regenerability can be expressed quantitatively in terms of a cycle length reduction associated with regeneration. For purposes of this specification and the appended claims, the xe2x80x9ccycle length reductionxe2x80x9d attributable to a first regeneration of the catalyst is the percentage reduction in cycle length that is observed when comparing first and second operating cycles obtained with the same catalyst charge when the two cycles are conducted under conditions that are both economically viable and essentially equivalent in terms of both average paraffin conversion and other operating conditions aside from catalyst activity. In such a pair of cycles, the first cycle begins with fresh catalyst, continues without regeneration, and ends at a time selected to provide an economically-viable combination of cycle length and average paraffin conversion. The second cycle begins after the first regeneration, continues without further regeneration, and ends at a time selected to provide an average paraffin conversion about the same as that for the first cycle. In each case, idle periods within cycles are not included in the calculation of cycle length or average conversion. A preferred and/or economically viable cycle length is readily determinable by one of ordinary skill operating a catalytic process, for a given set of circumstances.
As determined by the above method, the cycle length reduction attributable to a first regeneration of a catalyst according to the invention is preferably not greater than 50% of the length of the first cycle. Catalysts according to this invention in a more preferable embodiment are characterized as having a cycle length reduction attributable to a first regeneration of not greater than 35% of the length of the first cycle. Catalysts according to this invention according to a further more preferred embodiment are characterized as having a cycle length reduction attributable to a first regeneration of not greater than 20% of the length of the first cycle.
Considering a catalyst prepared in accordance with a preferred embodiment in which the primary catalytic component is Pt, it is well known that the conditions of catalyst preparation must be adjusted to provide a catalyst with adequate mechanical strength and a high degree of Pt dispersion. According to the invention, any distribution of Pt within the support can be employed, provided that it results in a favorable Pt dispersion that has adequate thermal stability. Preferably, the Pt distribution is as uniform as possible. More particularly, the highest local concentrations of the Pt must be kept low enough to avoid excessive agglomeration of the Pt under the conditions of catalyst use and regeneration. The purity of the alumina starting material and the conditions of catalyst preparation must also be adjusted to provide adequate thermal stability of the support structure in the finished catalyst; the stability must be sufficient to avoid excessive occlusion of the Pt during the use of the catalyst. Similar considerations also apply if other metals of the platinum group are used, and these considerations are known to those skilled in the preparation and use of supported catalysts.
In general, many methods useful for the preparation of catalysts comprising a platinum group metal supported on alumina may be used to produce catalysts according to this invention. However, the overall processing of the catalyst precursors must conform to the teachings herein to a sufficient degree to yield a finished catalyst material having the unique combination of physical characteristics and properties (including pore size distribution) that lie within the limits defined by the claims of this invention. The novel combination of properties possessed by the catalysts described herein is necessary to provide such a catalyst material having relatively high degrees of volumetric activity, selectivity, thermal stability, and regenerability. While a preferred preparative method according to the invention is described in Example II below, substantial variations in the method of preparation can be made without departing from the metes and bounds of the claimed invention, as will be appreciated by those of ordinary skill after reading this specification and its appended claims, since many of the general principles for preparing catalysts known in the art may be used to prepare the catalysts of the invention. In any case, it is preferred to select impregnation conditions that result in favorable distributions of added components within the support as well as a high degree of dispersion of the primary catalytic component.
Catalysts prepared in accordance with a preferred form of the invention include a support, a primary catalytic component, an acidity control component, and optionally an activator component. These components are often discussed herein only in terms of the elements involved, but it is to be understood that the elements cited may be present in various oxidation states or as components of various chemical compounds or complexes at different stages of the preparation or use of the catalysts.
Various known methods for combining catalyst supports with added catalyst components during catalyst preparation are appreciated by those of ordinary skill in the art. Any of these methods that are useful in the preparation of catalysts comprising one or more platinum group metals deposited upon a porous alumina support can be used to deposit added catalyst components (including soluble metallic species, whether complexed or uncomplexed) in the preparation of catalysts according to the present invention. For example, a suitable alumina catalyst support could be immersed in a solution containing one or more heat decomposable salts of metals to be employed. The activator component and the primary catalytic component can in one embodiment be satisfactorily deposited upon the catalyst support simultaneously by using a solution containing both components. In some instances, better results are obtained if the activator component is applied first, followed by a calcination step, with the calcined material thereafter being impregnated with a solution comprising the primary catalytic component. While it is also possible to apply the primary catalytic component first followed by application of the activator component, this procedure is not usually advantageous. As described in Example II below, excellent results were obtained by a method in which an activator component and an acidity control component were simultaneously added to the support in a first impregnation step, and the primary catalytic component was subsequently added separately in a second impregnation step. In general, it is most preferable to add the primary catalytic component after any calcination step that significantly reduces the surface area of the support, in order to reduce the risk of wasteful entrapment of some or all of the primary catalytic component by the support during calcination.
While any of the heat decomposable soluble salts of the metals to be deposited on the alumina support can be employed in accordance with the invention, the best results are usually obtained by the use of salts that do not include a halogen. Halogen containing salts such as chloroplatinic acid, while usable, are not usually especially preferred because their use results in the catalyst containing at least some halogen ions, and the presence of halogen ions in the catalyst material, even in small amounts, could potentially promote undesirable side reactions. Similarly, metal sulfates are usually not advantageous because the sulfate ion is removed from the catalyst material only with difficulty, and the presence of sulfate ions in the catalyst even in small amounts can be disadvantageous. The most preferred catalyst impregnation solutions containing Group I-B metals are solutions comprising nitrates such as copper nitrate or silver nitrate dissolved in aqueous ammonium hydroxide. The most preferred solutions containing platinum metals are those prepared by dissolving diammine dinitrites such as platinum diammine dinitrite, Pt(NH3)2(NO2)2 or palladium diammine dinitrite, Pd(NH3)2(NO2)2 in aqueous ammonium hydroxide.
When an aqueous ammoniacal solution of platinum diammine dinitrite is used (as in Examples I-III below), the following method of preparing the solution gives good results. Platinum diammine dinitrite is dissolved in a hot concentrated aqueous solution of ammonium hydroxide to form a homogeneous intermediate stock solution in which the Pt concentration is somewhat higher than that intended for use in catalyst impregnation. The stock solution may then optionally be held at a temperature in the range of about 65-85xc2x0 C. (xe2x80x9cagedxe2x80x9d) for a period of up to about four hours, resulting in some approach toward equilibrium of the distribution of complexes present in the solution. The amount of solution needed for catalyst impregnation is conveniently provided by diluting an appropriate quantity of intermediate stock solution with deionized water while maintaining a temperature suitable for maintaining homogeneity, such as about 65xc2x0 C. If desired, the diluted solution can be heated or cooled to a somewhat different temperature for the catalyst impregnation step. In any case, it is highly preferable that the solution is homogeneous when brought into contact with the catalyst support. It will be appreciated by those skilled in the art that the most preferred method of preparation may vary, depending, e.g., on the desired size of the catalyst particles. It belongs to the skill of the skilled person to select the most suitable method and solution concentrations for a given set of circumstances.
During a given impregnation step, the amount of solution employed can be varied widely. It is preferred to use a sufficiently large volume of solution to permit uniform deposition of the metal salts. A very satisfactory procedure has been found to be to use that volume of solution in each instance which is equal to the amount required to fully saturate the support material. According to this procedure, the amount of metal deposited upon the support during an impregnation step is simply equal to the entire amount of metal present in the solution that is used for the impregnation step. The amount of solution required to saturate the support material can be readily determined by tests conducted on a small sample of the support material. If a selected metal salt (such as a basic carbonate, as but one example) in any instance is not sufficiently soluble to permit the desired amount of the metal to be deposited in a single application, the metal can be applied in a plurality of steps, with the catalyst material being dried and/or calcined between such steps.
In various examples provided herein, calcination conditions are described in terms of a particular calcination temperature and either a specified calcination time or a calcination time sufficient to produce a stated effect. It is appreciated by those skilled in this art that somewhat different calcination temperatures could also be used with essentially equivalent results. In some cases, compensating adjustments of calcination time might be required, as such adjustments are well known in the art. When the calcination is carried out to achieve catalyst particles having a particular surface area, the progression of the change in surface area may be readily monitored by testing samples of the material at appropriate intervals using techniques known to those skilled in the art, such as the BET method with nitrogen as the adsorbed species. When treatment at elevated temperatures is employed to eliminate volatile or decomposable impregnation by-products such as nitrites, nitrates, ammonia, etc., similar testing of small quantities of catalyst pellets at selected intervals may be undertaken to assure success, or alternatively, those skilled in the art may employ with confidence those conditions well known and recognized as sufficient for such elimination, provided the finished catalyst possesses the physical limitations set forth in the claims herein. While calcination in air is indicated in various examples, the present invention also contemplates the use of other atmospheres during calcination, since the use of other atmospheres, including oxidizing, reducing, and inert atmospheres is known to artisans of ordinary skill in catalyst calcination. Calcination is conventionally carried out at a temperature between 300 and 1200 degrees Centigrade, preferably between 400 and 1000 degrees C, including every degree of temperature therebetween. The duration of calcination treatment may be any amount of time between 0.5 and 24 hours, but is preferably between forty five (45) minutes and 4 hours. It will be appreciated that the average temperature during the calcination will be higher than the average temperature during any drying treatment.